1. Field of the Invention
This invention relates to a liquid crystal device for displaying images by application of an electric field to a liquid crystal layer and more particularly, to liquid crystal device of a scattering mode without use of any polarizer. The invention also relates to a method for making such a liquid crystal device as mentioned above.
2. Description of the Related Art
Various display modes of liquid crystal devices making use of liquid crystals are known including a birefringence mode, a polarization mode, a scattering mode and the like. Of these, the device of the scattering mode is now being widely used owing to its simple arrangement.
The known liquid crystal devices utilizing the scattering mode of liquid crystals include those of the dispersion type, the ferrodielectric liquid crystal type and the dynamic dispersion type. These devices make no use of any polarizer and can be realized inexpensively, so that intensive studies have now been made.
Of the scattering modes utilized for display, the dispersion type is described in detail for convenience' sake. The liquid crystal device of the dispersion type includes the following three classes of arrangements.
The first class consists of a liquid crystal device wherein a liquid crystal substance is microcapsulated, the resultant microcapsules are added to a polymer material to form a liquid crystal layer, and the liquid crystal layer is provided between a pair of substrates which are facing each other.
This type is so arranged that a refractive index (n.sub.o) which is sensitized with light passing through the liquid crystal substance in the microcapsules along the direction of application of a voltage and a refractive index (n.sub.p) sensitized with light passing through the dispersion medium of the polymer material are substantially equal to each other (i.e. n.sub.o =n.sub.p). The light passing through the liquid crystal layer along the direction of application of a voltage at the time of the application of the voltage is allowed to pass substantially at an equal refractive index through both the liquid crystal substance in the microcapsules and the polymer dispersion medium in the liquid crystal layer. Eventually, the liquid crystal layer becomes optically transparent and permits transmission of the light.
At the time when no voltage is applied, the liquid crystal substance in the individual microcapsules suffers only a slight alignment regulation force from the wall surfaces of the microcapsules. In this condition, the liquid crystal molecules, respectively, turn in random directions, so that the light passing through the respective microcapsules are scattered. Thus, the liquid crystal layer becomes opaque or cloudy, not permitting passage of light therethrough.
The second class includes a liquid crystal device wherein a liquid crystal substance is added to and dispersed in a polymer material to form a liquid crystal layer. This layer is provided between a pair of substrates in face-to-face relation.
Like the first type, this type is so arranged that a refractive index (n.sub.o) of the dispersed liquid crystal substance through which light passes along the direction of application of a voltage and a refractive index (n.sub.p) of the dispersion medium made of the polymer material through which light passes are substantially equal to each other (i.e. n.sub.o =n.sub.p). Accordingly, light passing through the liquid crystal layer along the direction of application of a voltage at the time of the application of the voltage is subjected to refractive indices which are equal in both the liquid crystal substance and the polymer material in the liquid crystal layer. Thus, the liquid crystal layer becomes optically transparent, through which light can pass.
When no voltage is not applied to the layer, the liquid crystal substance suffers only a slight alignment regulation force at the interfaces with the polymer material serving as the dispersion medium. In this condition, the liquid crystal molecules, individually, turn in random directions, under which light passing through the liquid crystal substance is scattered, resulting in an opaque liquid crystal layer and not permitting transmission of the light.
The third class comprises a liquid crystal device as shown in FIG. 18. In the figure, a liquid crystal substance 7 is provided as a dispersion medium and fine particles 13 of an isotropic material are dispersed in the liquid crystal substance 7 to form a liquid crystal layer 9. The liquid crystal layer 9 is provided between a pair of substrates 3, 3, each made of a base 1 and a transparent electrode 2 formed on the surface of the base 1, thereby obtaining a liquid crystal device 14.
The transparent electrodes 2 which are vertically kept away from each other are connected to a power supply 11 through a switch 12, by which a necessary potential can be supplied to the liquid crystal layer 9 as desired. The transparent electrodes 2, 2 may be appropriately formed with a protective film 4 on each surface thereof.
In this type of device, a refractive index (n.sub.o) of the dispersed liquid crystal substance through which light passes along the direction of application of a voltage and a refractive index (n.sub.p) of the dispersed particles, which are fine particles 13 of an isotropic material, through which light passes are substantially equal to each other (i.e. n.sub.o =n.sub.p). Accordingly, when light irradiated from below the liquid crystal device passes through the liquid crystal layer along the direction of application of a voltage at the time of the application of the voltage, the light is subjected to refractive indices which are substantially equal to each other on passage through the liquid crystal substance 7 used as the dispersion medium and also through the fine particles 13 made of an isotropic material and used as the dispersed particles in the liquid crystal layer 9. Thus, the light is able to transmit through the liquid crystal layer, so that the liquid crystal layer becomes optically transparent.
In contrast, when no voltage is applied to the layer, the liquid crystal substance 7 used as the dispersion medium suffers only a slight alignment regulation force from the interfaces with the substrates 3, 3 provided at opposite sides and also from the interfaces with the fine particles 13 made of the isotropic material. The liquid crystal molecules, respectively, turn in random directions. As a result, the irradiation light is scattered during the passage through the liquid crystal layer 9, resulting in the liquid crystal layer being opaque.
Of these classes, the third class is the simplest in arrangement. Thus, intensive studies have been recently made for practical usage.
However, with the known arrangement of the liquid crystal device of the third class shown in FIG. 18, the fine particles 13 which are dispersed in the liquid crystal layer 9 as being fine in size and having different densities. This is disadvantageous in that the liquid crystal substance cannot be charged into a cell according to a known vacuum injection method because of the hindrance of the fine particles 13 in the liquid crystal material. Accordingly, when assembling the liquid crystal device 14 of this type, the liquid crystal substance 7 dispersing the fine particles 13 therein is dropped on the surface of one of the substrates 3, 3 provided with spacers 6 thereon, on which the other substrate 3 is placed.
However, the above procedure involves a difficulty in satisfactorily preventing incorporation of bubbles, resulting in a very poor yield.
In a long-term use, the dispersed fine particles 13 are liable to locally settle at the bottom by the action of gravity as is particularly shown in FIG. 19. Alternatively, as shown in FIG. 20, the dispersed fine particles 13 may coagulate with one another. This impedes reliability of the liquid crystal device in use over a long time.
In order to solve the above problem, we made intensive studies and proposed a liquid crystal device in our Japanese Patent Application No. 5-341436.
The liquid crystal device of the application is as shown in FIG. 21 and includes a liquid crystal layer 9 between a pair of substrates 3, 3 each having an electrode 2 on the main surface of the substrate 3. The liquid crystal layer 9 comprises a liquid crystal substance wherein the twisting of the molecules is appropriately controlled, and spacers 6 for keeping a given space between the substrates 3, 3. The device is characterized in that the substrates 3, 3, respectively, have on the surfaces thereof protrusions 17 made of a material which has a refractive index substantially equal to that of the liquid crystal substrate 7 relative to an ordinary or extraordinary ray.
With the arrangement of the liquid crystal device 18, when irradiation light 21 passes, as shown in FIG. 22A, through the liquid crystal layer 9 along the direction of application of a voltage at the time of the application of the voltage, the light is subjected to refractive indices of the liquid crystal substance 7 serving as a dispersion medium and the fine particles 17 made of an isotropic material and serving as dispersed particles, which are substantially equal to each other. Thus, the light is transmitted as light 22 through the liquid crystal layer 9 which is optically transparent.
In contrast, as shown in FIG. 22B, when no voltage is applied, the dispersion medium of the liquid crystal substance undergoes only a slight alignment regulation force from the interfaces with the substrates 3, 3 at opposite sides and also with the dispersed particles which are fine in size and are made of an isotropic material. Thus, the molecules turn in random directions. When the irradiation light 21 passes through the liquid crystal layer 9, it is scattered as 23, so that the liquid crystal layer 9 is observed as opaque.
Since the device of the above-stated type has protrusions 17 fixed at the surfaces of the substrates 3, 3, a liquid crystal substance can be charged into a cell according to any known vacuum injection method. The incorporation of bubbles can be completely prevented, with a yield being remarkably improved. When the device is used over a long time, the protrusions are not locally shifted owing to the action of gravity or are not coagulated with one another at all, ensuring the long-term use of the liquid crystal device 18. This permits a simple construction without use of an expensive polarizer and can reliably realize a liquid crystal device which make use of the dispersion mode of liquid crystals.
However, this device is disadvantageous in that, as shown in FIG. 16, a distance, d'LC, between the protrusions 17 of the opposite substrates and a distance, dLC, between the protrusion-free portions differ from each other and that the dielectric constants of the liquid crystal molecules and the material for the protrusions are different from each other (i.e. the dielectric constant, .epsilon.LC, of liquid crystal substances is approximately 18 and that, .epsilon.p, of a resin material ranges from 3 to 5, so that a potential exerted on the liquid crystal may differ depending on the portion being applied therewith. For instance, when a voltage at a protrusion-free portion is taken as E.sub.o, a voltage at a portion where protrusions 17 are formed at the opposite side substrates 17 is taken as ELC and the height of the protrusion is taken as Hp, the difference between the values of E.sub.o and ELC becomes greater at a greater value of Hp. This is particularly shown in FIG. 10. It will be noted that in FIG. 10, the distance, dLC is 8 .mu.m, the dielectric constant, .epsilon.LC, of a liquid crystal is 18 (marked as .circle-solid.), and the dielectric constant, .epsilon.p, of a resin constituting the protrusion is 3 (marked as .largecircle.).
When a non-uniform voltage is applied to the liquid crystal layer, the sharpness of the V/T characteristics lowers. This leads to, as shown in FIG. 17, a small width of effective hysteresis, .DELTA.H, in bistable drive, making the bistable drive substantially difficult.
Since the voltage exerted on the liquid crystal layer is not uniform, the liquid crystal display may not be uniform in some cases.